1. Field of the Invention
The present invention relates to ASK (Amplitude Sift Keying) modulators that are suitably applicable to wireless communication systems.
2. Description of Related Art
In performing a wireless data communication, an ASK modulation system in a wireless communication system transmits radio waves only during periods in which the transmission data is at a logic xe2x80x9cHxe2x80x9d level and halts a radio wave transmission during periods in which the transmission data is at a logic xe2x80x9cLxe2x80x9d level.
For example, modulation systems of wireless units of ETC (Electronic Toll Collection; non-stop automatic toll collection) systems adopt this system. Here, the ETC system performs a non-stop automatic toll collection at a tollbooth, wireless-communicating between a wireless beacon installed on the road side and an on-vehicle equipment. ETC systems use a high frequency band of 5.8 GHz.
FIG. 1 is a block diagram showing an exemplary configuration of a conventional ASK modulator that is applied to such an ETC system. In what follows, with reference to FIG. 1, the configuration and operation of the conventional ASK modulator will be explained.
As shown in FIG. 1, a 5.8 GHz band wireless carrier signal generated by the local oscillator 1 is amplified by the buffer amplifier 2 and is supplied to the input terminal IN of the gallium arsenic (GaAs) high speed switch 3. On the other hand, a data signal supplied to the data signal input terminal a is sent to the control input terminal CONT of the GaAs high speed switch 3.
FIG. 2 explains the operation characteristic of the GaAs high speed switch 3. In FIG. 2(A), when the data signal supplied to the control terminal CONT is a logic xe2x80x9cHxe2x80x9d signal, the logic xe2x80x9cHxe2x80x9d signal is input to the gate of the MOS-FET 302 via the resistor 310. As a result, the region between the drain and source of the MOS-FET 302 is turned on. On the other hand, when a logic xe2x80x9cLxe2x80x9d signal is input to the MOS-FET 305 via the inverse device 307 and resistor 306, the region between the drain and source of the MOS-FET 305 is turned off. As a result, the 5.8 GHz wireless carrier signal input to the input terminal IN is supplied to the output terminal OUT via the capacitance device 301, MOS-FET 302, and capacitance device 304.
When the data signal supplied to the control terminal CONT is a logic xe2x80x9cLxe2x80x9d signal, the logic xe2x80x9cLxe2x80x9d signal is supplied to the gate of the MOS-FET 302. As a result, the region between the drain and source of the MOS-FET 302 is turned off. On the other hand, a logic xe2x80x9cHxe2x80x9d signal is supplied to the gate of the MOS-FET 305. As a result, the region between the drain and source of the MOS-FET 305 is turned on. Therefore, the 5.8 GHz wireless carrier signal input to the input terminal IN is supplied to the ground GND via the capacitance device 301, MOS-FET 305, and capacitance device 308 in this order but not to the output terminal OUT.
Therefore, as shown in FIG. 2(B), the GaAs high speed switch 3 supplies the 5.8 GHz wireless carrier signal to the power amplification circuit 4 when the data signal sent from the data signal input terminal a is a logic xe2x80x9cHxe2x80x9d signal. On the other hand, when the data signal sent from the data signal input terminal a is a logic xe2x80x9cLxe2x80x9d signal, the GaAs high speed switch 3 does not supply the 5.8 GHz wireless carrier signal to the power amplification circuit 4.
By this operation, the GaAs high speed switch 3 outputs an ASK modulation signal in the 5.8 GHz band, that is, a 5.8 GHz band wireless carrier signal that intermittently operates by the same timings as the logic level of the data signal.
The power amplification circuit 4 then amplifies to a prescribed power level the ASK modulation signal in the 5.8 GHz band that has passed through the GaAs high speed switch 3. The band-pass filter 5 shown in FIG. 1 removes spurious radiation components of the ASK modulation signal, which are not needed for communication. The ASK modulation signal is then sent to an antenna not shown in the drawing via the antenna terminal 6 and is emitted as a radio wave.
FIG. 3(A) shows the waveform of the original signal to be supplied to the conventional ASK modulator. FIG. 3(B) shows the waveform of a Manchester signal transformed from the original signal to be supplied to the data signal input terminal of the conventional ASK modulator. FIG. 3(C) shows the waveform of a 5.8 GHz band ASK modulated signal generated by the conventional ASK modulator. In this example of the prior art, the original data signal is transformed into a Manchester data signal so that the clock component of the original data signal can be easily reproduced on the receiver side. The original data signal having the waveform shown in (A) is transformed into a Manchester data signal having the waveform shown in (B). When this data signal shown in (B) is supplied to the data signal input terminal a, the GaAs high speed switch 3 outputs an ASK modulation signal in the 5.8 GHz band shown in (C). This is a 5.8 GHz band wireless carrier signal that intermittently operates with exactly the same timings as the Manchester data signal shown in (B).
However, the above-described conventional ASK modulator has the following problem.
10xc3x97log10[(sin4(xcfx80f/2xc3x97106))/(xcfx80f/2xc3x97106)2]xe2x80x83xe2x80x83(1)
The above-provided expression (1) is a general formula of a data signal transformed into a Manchester signal. Data signals transformed into Manchester signals have rectangular waveforms as shown in FIG. 3. The frequency spectrum of such a signal extends to infinity.
In other words, the conventional ASK modulator transforms a data signal having a frequency spectrum that extends to infinity into a 5.8 GHz band wireless carrier signal and obtains an ASK modulated signal by intermittently operating this 5.8 GHz band wireless carrier signal. Therefore, the frequency spectrum of this ASK modulation signal is centered at 5.8 GHz and also spreads to infinity, which is a problem.
A band restricting low pass filter can be inserted into FSK (Frequency Shift Keying) modulators and PSK (Phase Shift Keying) modulators to restrict the frequency band of the data signal as well as the frequency band of the modulated 5.8 GHz band wireless carrier signal.
However, the GaAs high speed switch 3 of the conventional ASK modulator is turned on and off at about an intermediate voltage between the drain voltage and source voltage of the MOS-FET. Therefore, even if a band restricting low pass filter is inserted into the conventional ASK modulator to restrict the frequency band of the data signal, the frequency spectrum of the ASK modulated signal obtained on the output side of the GaAs high speed switch 3 is centered at 5.8 GHz and still spreads to infinity. Therefore, inserting a low pass filter into the conventional ASK modulator does not solve the problem.
For this reason, ASK modulators capable of reducing the frequency band width of the ASK modulation signal are in demand.
It is an object of the present invention to provide an ASK modulator to solve the problem.
To solve the problem, according to the first embodiment of the present invention, an ASK modulator which modulates an input signal and generates an amplitude shift keying (ASK) modulated signal in accordance with a logic level of the input signal is provided. This ASK modulator has an oscillating means for generating a carrier signal, a frequency filter means which passes a prescribed frequency component of the input signal and outputs a frequency limited signal having the prescribed frequency component, and a modulator which modulates the input signal based on the carrier signal and the frequency limited signal and outputs the ASK modulated signal having a limited frequency component.
According to one aspect of the first embodiment of the present invention, the frequency filter means may be a low pass filter which passes a low frequency component of the input signal.
According to another aspect of the first embodiment of the present invention, the modulator generates the ASK modulated signal having a limited frequency component by changing the amplitude level of the carrier signal so that the amplitude level of the carrier signal will be substantially linearly related with the level of the frequency limited signal.
According to further another aspect of the first embodiment of the present invention, the modulator has a voltage-current conversion means which generates a current signal having a current level that is substantially linearly related with the voltage level of the frequency limited signal such that the modulator generates the ASK modulated signal having a limited frequency component based on the current level of the current signal generated by the voltage-current conversion means.
According to further another aspect of the first embodiment of the present invention, the modulator has a pin diode which changes a resistance level of the pin diode in accordance with the level of a current supplied to the pin diode such that the modulator generates the ASK modulated signal having a limited frequency component based on the resistance level of the pin diode changed by the current level of the current signal that flows through the pin diode.
According to further another aspect of the first embodiment of the present invention, the modulator has a power amplifier which amplifies the amplitude level of the carrier signal to a prescribed level based on the current level of the current signal.
According to further another aspect of the first embodiment of the present invention, the modulator has a double balance mixer which mixes the carrier signal with the frequency limited signal and adjusts the amplitude level of the carrier signal so that the amplitude level of the carrier signal will be substantially linearly related with the level of the frequency limited signal.
According to further another aspect of the first embodiment of the present invention, the double balance mixer has a transformer and a diode.
According to further another aspect of the first embodiment of the present invention, the modulator has a power amplifier which amplifies the amplitude level of the carrier signal to a prescribed level based on the voltage level of the frequency limited signal.
According to further another aspect of the first embodiment of the present invention, the modulator has a negative voltage conversion means which converts the voltage level of the frequency limited signal to a negative level to produce a converted frequency limited signal and supplies the converted frequency limited signal to the power amplifier.
According to further another aspect of the first embodiment of the present invention, the modulator has a power amplification unit which amplifies the carrier signal to a prescribed power level to produce an amplified carrier signal and superposes the amplified carrier signal on the frequency limited signal.
According to further another aspect of the first embodiment of the present invention, the modulator has a superposition power amplifier which amplifies the frequency limited signal to enable the frequency limited signal to be superposed on the carrier signal that has been amplified to the prescribed power level by the power amplification unit.
According to further another aspect of the first embodiment of the present invention, the modulator has a power amplification unit which amplifies the carrier signal to a prescribed power level, a superposition power amplifier which amplifies the frequency limited signal to enable the frequency limited signal to be superposed on the carrier signal that has been amplified to the prescribed power level by the power amplification unit, and a last-stage ASK modulation circuit which generates the ASK modulated signal by superposing the carrier signal amplified by the power amplification unit on the frequency limited signal amplified by the superposition power amplifier.
According to further another aspect of the first embodiment of the present invention, the modulator adjusts said envelope of the carrier signal so that the envelope of the carrier signal will be substantially linearly related with the level of the carrier signal.
According to the second embodiment of the present invention, an ASK modulation method for modulating an input signal and generating an amplitude shift keying (ASK) modulated signal in accordance with a logic level of the input signal is provided. This ASK modulation method has the steps of generating a carrier signal, passing a frequency limited signal having a prescribed frequency component of the input, and modulating the input signal based on the carrier signal and the frequency limited signal to generate the ASK modulated signal having a limited frequency component.
According to one aspect of the second embodiment of the present invention, the passing step has a step of passing a low frequency band of the input signal.
According to another aspect of the second embodiment of the present invention, the modulating step has a step of changing an amplitude level of the carrier signal so that the amplitude level of the carrier signal will be substantially linearly related with the level of the frequency limited signal.
According to the third embodiment of the present invention, an on-vehicle data transmitter for transmitting a data signal used for paying a toll is provided. This data transmitter has a signal processing unit which outputs data to be used for paying the toll, an ASK modulator which generates an amplitude shift keying (ASK) modulated signal in accordance with the logic level of the data signal, and an antenna for transmitting the ASK modulated signal modulated by the ASK modulator. This ASK modulator has an oscillating means for generating a carrier signal, a low pass filtering means which outputs a frequency limited signal having a prescribed low frequency component of the input signal, and a modulator which modulates the input signal based on the carrier signal and the frequency limited signal and outputs the ASK modulated signal having a limited frequency component.
According to one aspect of the third embodiment of the present invention, the modulator generates the ASK modulated signal having a limited frequency component by changing the amplitude level of the carrier signal so that the amplitude level of the carrier signal will be substantially linearly related with the level of the frequency limited signal.
According to the fourth embodiment of the present invention, a wireless beacon for transmitting and receiving a data signal is provided. This wireless beacon has a signal processing unit for processing a transmission data signal, an ASK modulator for generating an ASK modulated data signal in accordance with the logic level of the transmission data signal, and an antenna for transmitting to a communication partner the transmission data signal ASK modulated by the ASK modulator.
Moreover, in order to solve the problem, according to another embodiment of the present invention, an ASK modulator, which has an oscillation means for generating a carrier signal, for generating an ASK modulated signal whose amplitude has been changed in accordance with the logic level of the input signal is provided. This ASK modulator has (1) a frequency filter means which passes only a prescribed frequency component of the input signal and outputs a frequency limited signal having a prescribed frequency component, and (2) a modulator which adjusts the envelope of the carrier signal supplied from the oscillating means so that the envelope of the carrier signal will be substantially linearly related with the level of the frequency limited signal supplied from the frequency filter means.
According to an embodiment of the present invention, the envelope of the carrier signal is changed so that it will be linearly related to a level of the frequency limited signal obtained from the input signal by passing only a low frequency component of the input signal. Thus, the spread of the frequency band of the ASK modulated signal is confined within a frequency band determined by the filter means.
According to another embodiment of the present invention, an ASK modulator, which has an oscillation means for generating a carrier signal, for generating an ASK modulated signal whose amplitude has been changed in accordance with the logic level of the input signal is provided.
This ASK modulator has (1) a frequency filter means which passes only a prescribed frequency band of the input signal and outputs a frequency limited signal having a prescribed frequency component, and (2) a power amplification modulator which changes the envelope of the carrier signal so that it will be linearly related to the level of the frequency limited signal by amplifying the carrier signal to a prescribed power level while changing the amplification factor in accordance with the level of the frequency limited signal supplied from the frequency filter means.
According to this embodiment of the present invention, by changing the envelope of the carrier signal so that it will be linearly related to the level of the frequency limited signal by amplifying the carrier signal to a prescribed power level while changing the amplification factor in accordance with the level of the frequency limited signal supplied from the frequency filter means that has passed only the low frequency band of the input signal, the spread of the frequency band of the ASK modulated signal is confined within a frequency band determined the filter means.
According to further another embodiment of the present invention, an ASK modulator, which has an oscillation means for generating a carrier signal and a power amplifier for amplifying this carrier signal to a prescribed power level, for generating an ASK modulated signal whose amplitude has been changed in accordance with the logic level of the input signal is provided.
This ASK modulator has a (1) a frequency filter means which passes only a prescribed frequency band of the input signal and outputs a frequency limited signal having a prescribed frequency component, (2) superposition power amplifier which amplifies the power level of the frequency limited signal supplied from the frequency filter means to a level at which the frequency limited signal can be superposed on another signal, and (3) a superposition modulator, which is installed at the last stage of the power amplifier, for changing the envelope of the carrier signal so that it will be linearly related to the level of the frequency limited signal by superposing the frequency limited signal supplied from the power amplifier onto the carrier signal supplied from the superposition power amplifier.
According to this embodiment of the present invention, the envelope of the carrier signal is changed so that it will be linearly related to the level of the frequency limited signal by superposing the frequency limited signal onto the carrier signal. As a result, the spread of the frequency band of the ASK modulated signal is confined within a frequency band determined the filter means.
The outline of the above-described invention does not list all the required characteristics of the present invention. Sub-combinations of these characteristics group are also covered by the scope of the present invention.